Non-intrusive MPSC node-based queue

+ Waitfree and fast producers. One XCHG is maximum what one can get with multi-producer non-distributed queue.
+ Extremely fast consumer. On fast-path it's atomic-free, XCHG executed per node batch, in order to grab 'last item'.
+ No need for node order reversion. So pop operation is always O(1).
+ ABA-free.
+ No need for PDR. That is, one can use this algorithm out-of-the-box. No need for thread registration/deregistration, periodic activity, deferred garbage etc.

- Push function is blocking wrt consumer. I.e. if producer blocked in (*), then consumer is blocked too. Fortunately 'window of inconsistency' is extremely small - producer must be blocked exactly in (*). Actually it's disadvantage only as compared with totally lockfree algorithm. It's still much better lock-based algorithm.
- The algorithm is not linearizable.

struct mpscq_node_t
    mpscq_node_t* volatile  next;
    void*                   state;


struct mpscq_t
    mpscq_node_t* volatile  head;
    mpscq_node_t*           tail;

void mpscq_create(mpscq_t* self, mpscq_node_t* stub)
    stub->next = 0;
    self->head = stub;
    self->tail = stub;

void mpscq_push(mpscq_t* self, mpscq_node_t* n)
    n->next = 0;
    mpscq_node_t* prev = XCHG(&self->head, n); // serialization-point wrt producers, acquire-release
    prev->next = n; // serialization-point wrt consumer, release

mpscq_node_t* mpscq_pop(mpscq_t* self)
    mpscq_node_t* tail = self->tail;
    mpscq_node_t* next = tail->next; // serialization-point wrt producers, acquire
    if (next)
        self->tail = next;
        tail->state = next->state;
        return tail;
    return 0;